£44" PRINCIPLES'OF CHEMISTRY

This is accompanied by a 'change of colour and development of heat,
Pb02 f S02 = PbS04. When triturated with sulphur the mixture
explodes, the sulphur burning at the expense of the oxygen of the lead
dioxide. Tetrachloride of lead, PbCl4, belongs to the same class o$<
lead compounds as PbOa. This chloride is formed by the action of
strong hydrochloric acid upon Pb02, or, in the cold, by passing a stream
of chlorine through water containing PbCl2 in suspension. The resul-
tant yellow solution gives off chlorine when heated. With a solution
of sal ammoniac (Nicolukin, 1885) it gives a precipitate of a double
salt, (NH4)2Pb016 (very slightly soluble in a solution of sal ammoniac),
which when treated with strong sulphuric acid (Friedrich, 1890) gives
PbCl4 as a yellow liquid sp. gr. 3-18, which solidifies at — 18°, and when
heated gives PbCl2 + C12. It is not acted upon by H2S04 like SnCl4.
Tetrafhioiide of lead (Brauner) belongs to the same class of compounds,
it easily forms double salts and decomposes with the evolution of
fluorine (Chapter IL, Note 49 bis), ^w*

Amongst the elements of the second and third groups it was
observed that the elements were more basic in the even than in the
uneven series. It is sufficient to remember calcium, strontium, and
.barium in the even, and magnesium, zinc, and cadmium in the uneven
series. In addition to this, in the even series, as the atomic weight
Increases, in the sam'e type of oxidation the basic properties increase
(the acid properties decrease); for example, in the second group, calcium,
strontium, barium. The same also appears in the fourth and all the
following groups. In the-ueven series of the fourth group titanium,'
zirconium, cerium, and thorium are found. All their highest oxides,
B02, even the lightest, titanic oxide, Ti02, have more highly developed
basic properties than silica, Si02, and in addition to this the basic
properties are more distinctly seen in zirconium dioxide, ZrO2, than
in. titanic oxide, Ti02, although the acid property of combining with
bases still remains. In the heaviest oxides, cerium dioxide, CeO2, and
thorium dioxide, ThOa, no acid properties are observed, these being
both purely b£sic oxides. In Chapter XVII. (Note 43) we already
pointed out tikis higher oxide of cerium. As the above-mentioned
elements are r'&ther rare in nature, have but little practical applica-
tion, and do not present any new forms of combination, it is un«
advisable to dwell on them in this treatise.

Titanium is found in nature in the form of its'anhydride or oxide,
Ti02, mixed with silicon in many minerals, but the oxide is also found

«e bi« ihe plumbatea of Ca ond|other similar metals, mentioned in Chapter III« Note 7»
also belong to the form PbXj. . ,. - fit PbP4f obtained by